docs: def indices2nodes(self, indices):
          """Return |Nodes| for these indices.
  
          Args:
              indices (tuple[int]): The indices in question.
  
          Returns:
              tuple[Node]: The |Node| objects corresponding to these indices.
  
          Raises:
              ValueError: If requested indices are not in the subsystem.
          """
          if set(indices) - set(self.node_indices):
              raise ValueError(
                  "`indices` must be a subset of the Subsystem's indices.")
          return tuple(self._index2node[n] for n in indices)